In the context of automatic stock-keeping and stock management of objects, bar codes are often used for identifying the objects, since they are cost-effective to print and can be applied using printed labels. A major component of automatic systems for stock-keeping and stock management of objects is taking inventory after power-up, after manual interventions and in the context of special handling etc. For this purpose, in the above-mentioned systems, all storage locations required for taking inventory are automatically approached, for example using a robot which comprises a bar code reading device, and it is subsequently attempted to read the respective bar code of an object. Initially, it needs to be distinguished whether or not there is an object at the corresponding storage location. If there is an object of which the bar code can be identified, the corresponding object is linked, by way of the bar code thereof, to the corresponding storage location in the stock management system. Often, the stock management system is a control unit of the robot or is connected to said unit. Generally, it is a memory area in which the detected bar codes are stored in connection with a storage location. There are basically the following possible situations:    a) the object in the storage location is provided with a bar code which can be identified, and subsequently this bar code is stored in the stock management system;    b) there is an object in the storage location without a bar code, or the bar code cannot be detected, for example because it is incompatible or damaged etc.; and    c) there is no object in the storage location.
If at the corresponding storage location there is an object of which the bar code cannot be detected or which does not have a bar code, the aim is to detect whether the corresponding storage location is occupied by an unknown object.
If there is no object at the corresponding storage location, this storage location should be linked as a free storage location in the management system. Since the storage is often dark and the objects are uniform articles, an illumination unit of the bar code reading device is required. This is often provided with a laser light or an IR or LED light source to illuminate the objects. Bar code reading devices based on photosensitive media, such as CCD-based or CMOS-based cameras or line scan cameras, are constructed in a correspondingly simple manner for detecting bar codes. There is no image recognition in the conventional sense. For reasons of cost, a unit should be used which is basically only targeted at detecting bar codes. Because of the very high packing density in a storage location, in some applications the free storage locations match the uniform objects virtually exactly. Also, the objects are provided in the same physical format, although the colours thereof may vary. A specific field of use are memory robots or tape robots which have one or more memory drives in which the storage media are to be transported, and when they have been written with data, the tapes have to be transported back to the storage location thereof again. Memory robots of this type are disclosed in greater detail in DE 10 2005 057 673, in DE 10 2005 054 560 B3, and in U.S. Pat. No. 7,184,242 B1, for example.
U.S. Pat. No. 5,450,385 A discloses a system which provides storage locations or information memory elements with machine-readable markings, for example a special bar code, which is located at a position inside the storage location which would be covered by an object if the storage location were occupied by an object.
U.S. Pat. No. 5,814,171 A likewise discloses a system which provides storage locations/receiving devices for storage with optically readable markings or imprints in the receiving device, which can only be read if the receiving device is empty, since they would be covered by a stored object.
DE 600 29 668 T2 discloses a method and a device, by way of which it can be determined using an optical detection system whether there are objects at predetermined positions. For this purpose, the storage locations which represent the positions are provided on the rear faces thereof with reference surfaces, which produce a contrast with the rest of the surface of the rear wall of a receiving device in terms of light reflection.
Drawbacks of the aforementioned systems and methods recognized in an embodiment of the present invention are that they:                i) absolutely require reflection even for empty storage locations, meaning that they are unsuitable for open shelf systems for example, since these reflections are provided by markings on the rear wall of the storage location or by the rear wall itself.        ii) require more complex method steps in the production of the systems, since corresponding empty marks have to be applied to the rear walls of the systems.        
U.S. Pat. No. 5,637,854 A discloses a device and a method for reading bar codes on test tubes. The presence of elements is detected on the basis of evaluating the spectrum of the reflected light. This system is provided for use in open systems without a rear wall, but is based on coherent light sources such as lasers. However, for automated use, laser-based bar code reading devices have the drawback that they cannot read omnidirectionally. This would accordingly make it necessary always to have bar codes applied in the correct orientation, or to track the bar code reading device in a plurality of directions. Further, bar code reading devices of this type do not have the option of interpreting multi-dimensional bar codes, and this is increasingly disadvantageous for many applications. The size of these devices can also only be miniaturised under some circumstances, and so laser-based bar code reading devices sometimes cannot meet the necessary requirements.